/Engine/Source/Runtime/SlateCore/Public/Widgets/SWindow.h
Popups, tooltips, drag and drop decorators all can be executed without creating a new window. This slot along with theSWindow::AddPopupLayerSlot()API enabled it.
struct FPopupLayerSlot : public [TSlotBase< FPopupLayerSlot >](API\Runtime\SlateCore\TSlotBase)